The invention relates to a resistive superconducting current limiter device comprising a conductor track composed of a strip-type superconductor, the conductor construction of which contains at least a substrate strip composed of a normally conducting substrate metal, a superconducting layer composed of an oxidic high-Tc superconductor material of the AB2CU3Ox type where A denotes at least one rare earth metal including yttrium and B denotes at least one alkaline earth metal, a buffer layer arranged in between, the buffer layer being composed of an insulating, oxidic buffer material, and also a cover layer applied on the superconducting layer, the cover layer being composed of a normally conducting cover layer material. A corresponding current limiter device is evident from DE 199 09 266 A1.
Superconducting metal oxide compounds having high critical temperatures Tc of above 77 k have been known since 1986, and they are therefore also referred to as high-Tc superconductor materials or HTS materials and are in particular a liquid nitrogen (LN2) cooling technique. Such metal oxide compounds include in particular cuprates based on special material systems such as, for example, of the type AB2Cu3Ox, where A is at least one rare earth metal including yttrium and B is at least one alkaline earth metal. The main representative of the material system of the so-called 1-2-3-HTS type is the so-called YBCO (Y1Ba2Cu3Ox where 6.5≦x≦7).
It is attempted to deposit the known HTS material on different substrates for various application purposes, superconductor material with the maximum possible phase purity generally being sought. Thus, in particular metallic substrates are provided for conductor applications (cf. e.g. EP 0 292 959 A1).
In a corresponding conductor construction, the HTS material is generally not deposited directly on a carrier strip serving as a substrate; rather, this substrate strip is firstly covered with at least one thin interlayer, which is also referred to as a buffer layer. This buffer layer, with a thickness of the order of magnitude of 1 μm, is intended on the one hand to prevent the diffusion of metal atoms from the substrate into the HTS material, which metal atoms could impair the superconducting properties. On the other hand, the buffer layer is intended to enable a textured formation of the HTS material. Corresponding buffer layers generally comprise oxides of metals such as zirconium, cerium, yttrium, aluminum, strontium or magnesium or mixed crystals comprising a plurality of the metals and are therefore electrically insulating. In a corresponding current-conducting conductor track, this results in problems as soon as the superconducting material undergoes transition to the normally conducting state (so-called “quenching”). In this case, the superconductor initially becomes resistive in sections and thus assumes a resistance R, for example by virtue of the fact that it is heated above the critical temperature Tc of its superconductor material (so-called “hot spots” or partial quenching regions) and is usually heated further, with the result that the layer can burn out.
On account of these problems, it is known to apply directly on the HTS conduction layer an additional metallic cover layer composed of a material that is compatible with the HTS material and has good electrical conductivity, such as Au or Ag, as a shunt against a burn-out. The HTS material is therefore in an electrically conductive, areal contact with the metallic cover layer (cf. DE 44 34 819 C).
Owing to the hot spots or partial quenching regions that are also present with shunts, the voltage is distributed nonuniformly along the superconductor layer. In the substrate strip carrying the superconducting layer, by contrast, the voltage U applied to the ends is dropped uniformly over the entire length or it is at an undefined intermediate potential if the ends are insulated from the applied voltage. Under certain circumstances, this may result in voltage differences in the conductor track across the buffer layer with respect to the substrate. Owing to the small thickness of this layer, this inevitably leads to electrical breakdowns and thus to the buffer layer, and possibly the superconducting layer, being destroyed at certain points. Voltages of the order of magnitude of 20 to 100 volts for buffer layer thicknesses of 1 μm typically suffice for a breakdown. A corresponding problem arises in particular when the intention is to create resistive current limiter devices with corresponding conductor strips. This is because in such a device the transition from the superconducting state to the normally conducting state is utilized to limit current in the event of a short circuit. In this case it is not readily possible to provide the buffer layer with a sufficient dielectric strength to withstand the operating voltages in the kV range which are customary for such devices.
A strip-type superconductor having a corresponding construction is used in the current limiter device that can be gathered from the DE-A1 document cited in the introduction. In the case of this construction, there is the risk discussed of electrical breakdowns across the buffer layer.